DESCRIPTION

These macros define and operate on three types of data structures:
lists, tail queues, and circular queues.
All three structures support the following functionality:

*

Insertion of a new entry at the head of the list.

*

Insertion of a new entry after any element in the list.

*

Removal of any entry in the list.

*

Forward traversal through the list.

Lists are the simplest of the three data structures and support
only the above functionality.

Tail queues add the following functionality:

*

Entries can be added at the end of a list.

However:

1.

All list insertions and removals must specify the head of the list.

2.

Each head entry requires two pointers rather than one.

3.

Code size is about 15% greater and operations run about 20% slower
than lists.

Circular queues add the following functionality:

*

Entries can be added at the end of a list.

*

Entries can be added before another entry.

*

They may be traversed backward, from tail to head.

However:

1.

All list insertions and removals must specify the head of the list.

2.

Each head entry requires two pointers rather than one.

3.

The termination condition for traversal is more complex.

4.

Code size is about 40% greater and operations run about 45% slower
than lists.

In the macro definitions,
TYPE
is the name of a user-defined structure,
that must contain a field of type
LIST_ENTRY,
TAILQ_ENTRY,
or
CIRCLEQ_ENTRY,
named
NAME.
The argument
HEADNAME
is the name of a user-defined structure that must be declared
using the macros
LIST_HEAD,
TAILQ_HEAD,
or
CIRCLEQ_HEAD.
See the examples below for further explanation of how these
macros are used.

Lists

A list is headed by a structure defined by the
LIST_HEAD
macro.
This structure contains a single pointer to the first element
on the list.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the list.
New elements can be added to the list after an existing element or
at the head of the list.
A
LIST_HEAD
structure is declared as follows:

LIST_HEAD(HEADNAME, TYPE) head;

where
HEADNAME
is the name of the structure to be defined, and
TYPE
is the type of the elements to be linked into the list.
A pointer to the head of the list can later be declared as:

struct HEADNAME *headp;

(The names
head
and
headp
are user selectable.)

The macro
LIST_ENTRY
declares a structure that connects the elements in
the list.

The macro
LIST_INIT
initializes the list referenced by
head.

The macro
LIST_INSERT_HEAD
inserts the new element
elm
at the head of the list.

The macro
LIST_INSERT_AFTER
inserts the new element
elm
after the element
listelm.

Tail queues

A tail queue is headed by a structure defined by the
TAILQ_HEAD
macro.
This structure contains a pair of pointers,
one to the first element in the tail queue and the other to
the last element in the tail queue.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the tail queue.
New elements can be added to the tail queue after an existing element,
at the head of the tail queue, or at the end of the tail queue.
A
TAILQ_HEAD
structure is declared as follows:

TAILQ_HEAD(HEADNAME, TYPE) head;

where
HEADNAME
is the name of the structure to be defined, and
TYPE
is the type of the elements to be linked into the tail queue.
A pointer to the head of the tail queue can later be declared as:

struct HEADNAME *headp;

(The names
head
and
headp
are user selectable.)

The macro
TAILQ_ENTRY
declares a structure that connects the elements in
the tail queue.

The macro
TAILQ_INIT
initializes the tail queue referenced by
head.

The macro
TAILQ_INSERT_HEAD
inserts the new element
elm
at the head of the tail queue.

The macro
TAILQ_INSERT_TAIL
inserts the new element
elm
at the end of the tail queue.

The macro
TAILQ_INSERT_AFTER
inserts the new element
elm
after the element
listelm.

Circular queues

A circular queue is headed by a structure defined by the
CIRCLEQ_HEAD
macro.
This structure contains a pair of pointers,
one to the first element in the circular queue and the other to the
last element in the circular queue.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the queue.
New elements can be added to the queue after an existing element,
before an existing element, at the head of the queue, or at the end
of the queue.
A
CIRCLEQ_HEAD
structure is declared as follows:

CIRCLEQ_HEAD(HEADNAME, TYPE) head;

where
HEADNAME
is the name of the structure to be defined, and
TYPE
is the type of the elements to be linked into the circular queue.
A pointer to the head of the circular queue can later be declared as:

struct HEADNAME *headp;

(The names
head
and
headp
are user selectable.)

The macro
CIRCLEQ_ENTRY
declares a structure that connects the elements in
the circular queue.

The macro
CIRCLEQ_INIT
initializes the circular queue referenced by
head.

The macro
CIRCLEQ_INSERT_HEAD
inserts the new element
elm
at the head of the circular queue.

The macro
CIRCLEQ_INSERT_TAIL
inserts the new element
elm
at the end of the circular queue.

The macro
CIRCLEQ_INSERT_AFTER
inserts the new element
elm
after the element
listelm.

The macro
CIRCLEQ_INSERT_BEFORE
inserts the new element
elm
before the element
listelm.

The macro
CIRCLEQ_REMOVE
removes the element
elm
from the circular queue.

CONFORMING TO

Not in POSIX.1-2001.
Present on the BSDs.
The
queue functions first appeared in
4.4BSD.

COLOPHON

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